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1.8: Evaluate Variable Expressions with Given Values

Difficulty Level: Basic Created by: CK-12
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Did you know that you can evaluate a variable expression for a given value? Have you ever been shopping and had to use math in the store? Take a look at this dilemma.

Hanson went shopping and bought cashews and almonds for his Mom. The cashews were $4.99 per pound, and the almonds were $3.50 per pound. Hanson looked at the prices and wrote down the following variable expression.

4.99x + 3.50y

If Hanson bought four pounds of cashews and four pounds of almonds, can you figure out how much money he spent? Use the information in this Concept to help you to evaluate this variable expression.

Guidance

Before you can learn how to evaluate a variable expression with a given value, you need to know how to identify a variable expression.

What is a variable expression?

A variable expression is a group of numbers, operations and variables without using an equal sign. A variable is a letter used to represent an unknown quantity. A constant is a number without a variable.

Here is a variable expression.

6a+7

In this variable expression, a is the variable and 7 is the constant .

Let’s look at evaluating variable expressions when we have been given a value for the variable.

Evaluate the expression 4g + 1.5 for g = 8 .

Step 1: Substitute 8 for the variable “ g .”

4(8) + 1.5

Step 2: Follow the standard order of operations to solve: parentheses, exponents, multiply, divide, add, and then subtract.

& 4(8) + 1.5 \ (\text{Multiply})\\& 32 + 1.5 \ (\text{Add})\\& 33.5

Our answer is 33.5

Evaluate the expression 5ab + 2a - 7 , when a = 2 and b = 4 .

Yes, there are. But don’t let that take you off course. If you simply substitute the given values into the expression and use the order of operations, you will end up with the correct answer.

Step 1: Substitute 2 for the variable “ a ” and 4 for the variable “ b .”

& 5ab + 2a - 7\\& 5(2)(4) + 2(2) - 7

Step 2: Follow the standard order of operations to solve: parentheses, exponents, multiply, divide, add, and then subtract.

& 5(2)(4) + 2(2) - 7 \ (\text{Multiply})\\& 10(4) + 4 - 7\\& 40 + 4 - 7 \ (\text{Add})\\& 44 - 7 \ (\text{Subtract})\\& 37

The answer is 37.

Example A

Evaluate the expression 5a + 2b - 17 , when a = 3 and b = 4

Solution: 6

Example B

Evaluate the expression 6xy + 2x - 7 , when x=4 and y=5

Solution: 121

Example C

Evaluate the expression 9x + 18y + 5 , when x=-6 and y=2

Solution: -13

Now let's go back to the dilemma from the beginning of the Concept. Here is the variable expression that Hanson wrote.

4.99x + 3.50y

We know that he bought 4 pounds of cashews and 4 pounds of almonds. Let's substitute those values into the expression for x and y .

4.99(4) + 3.50(4)

Now we can solve for the total cost.

19.96 + 14.00 = $33.96

This is our final answer.

Vocabulary

Variable Expression
a group of numbers, operations and variables without an equal sign.
Variable
a letter used to represent an unknown number
Constant
a number in an expression that does not have a variable.

Guided Practice

Here is one for you to try on your own.

Evaluate 4x + 3y - 18xy , when x=-3,y=4

Solution

First, substitute the given values into the expression for x and y .

4(-3) + 3(4) - 18(-3)(4)

Now we can evaluate the expression using the order of operations.

-12 + 12 - -216

0 + 216 = 216

This is our final answer.

Video Review

Khan Academy Evaluating Variable Expressions

Practice

Directions: Evaluate each variable expression using the given values for each variable.

  1. 6a+7 when a is 8
  2. 9x-y when x is 2 and y is 4.
  3. 5a+ b^2 when a is 12 and b is 4.
  4. \frac{8}{x}+2 when x is 4
  5. 6x+2.5 when x is 2.
  6. y^2+4 when y is 9
  7. 7x+2y when x is 3 and y is 5
  8. 9xy+ x^2 when x is 4 and y is 2
  9. 3ab+ b^3 when a is 9 and b is 2
  10. 16xy^2+ 14 when x is 3 and y is 4

11. 6xy + 4x when x is 2 and y is 7

12. 16y + 8xy when x is 3 and y is 4

13. 3x^2 + 24y when x is 3 and y is 4

14. -xy + 8xy when x is 3 and y is 4

15. 22x + 4y - 3xy when x is 3 and y is 4

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Basic

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Date Created:

Dec 19, 2012

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Aug 05, 2014
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